The present invention relates to a rubber composition for a tire tread and a pneumatic tire using the rubber composition.
In recent years, concern is increasing regarding a rise in oil prices due to problems of supply and the exhaustion of oil resources. Also, from the viewpoint of environmental issues such as resource conservation and the tightening of regulations for the prevention of carbon dioxide exhaust, the use of natural resources is being reconsidered. The tire industry is not an exception and as an alternative material to synthetic rubber, natural rubber is attracting attention. Natural rubber has high mechanical strength and excellent abrasion resistance and therefore is frequently used for large tires such as truck and bus tires. However, because natural rubber only has methyl groups with a small molecular weight in the side chains and has a low glass transition temperature (Tg) of −60° C., there is the problem that gripping performance is poor. Also, because natural rubber is a natural resource, there is the problem that ozone resistance, heat-aging resistance and weatherability are poor.
In order to solve these problems, natural rubber derivatives such as cyclized natural rubber, chlorinated natural rubber and epoxidized natural rubber are used. For example, JP-A-6-220254, JP-A-7-90123, JP-A-7-149955 and JP-A-2001-233995 suggest the method of using epoxidized natural rubber as the tire material. Herein, epoxidized natural rubber refers to natural rubber in which the unsaturated double bonds thereof are epoxidized. Epoxidized natural rubber has higher glass transition temperature (Tg) than natural rubber and is superior in mechanical strength, abrasion resistance and gas permeability, as the molecular cohesive force increases due to the epoxy groups, which are polar groups. Particularly, in rubber compositions which contain silica, silanol groups of the silica surface and epoxy groups of the epoxidized natural rubber are known to interact and as a result, mechanical strength and abrasion resistance equivalent to those obtained by a compound in which carbon black is added can be obtained.
However, although epoxidized natural rubber has a large hysteresis loss and excellent wet grip performance, there is the problem that rolling resistance increases (rolling resistance properties decrease) even when containing silica. Furthermore, because interaction between silica and epoxidized natural rubber is strong, in a blend compound of epoxidized natural rubber and other diene rubber, silica tends to be present on the epoxidized natural rubber side and as a result, there is the problem that not only does processability decrease, but also, hardness increases and abrasion resistance and heat-aging resistance decrease.
JP-A-11-71479 describes the method of compounding a specific stearic acid derivative. By using a specific stearic acid derivative as a processing aid for rubber components such as natural rubber and isoprene rubber, the rubber is softened and dispersibility of silica and other compounding agents becomes favorable. As a result, processability and abrasion resistance are improved. However, epoxidized natural rubber is not used as the rubber component in this method and processability and abrasion resistance are insufficient.